Methods and apparatus for controlled solid oxide fuel cell (SOFC)/turbine hybrid power generation

ABSTRACT

A method for operating a pressurized solid oxide fuel cell/turbine (SOFC/turbine) hybrid power generation system in which the hybrid power generation system includes an SOFC generator and a turbine generator includes controlling airflow to the SOFC/turbine hybrid power generation system in accordance with power demand and utilizing electrical current drawn from the SOFC generator to regulate SOFC generator temperature.

BACKGROUND OF THE INVENTION

This invention relates generally to methods and apparatus for controlledpower generation, and more particularly to methods and apparatus forcontrolled fuel cell/turbine hybrid power generation

At least one known power system integrates a solid oxide fuel cell(SOFC) generator, which can comprise a single cell or a stack of cells,an unreacted fuel combustor that can be integrated into the SOFCgenerator or provided as a stand-alone device, and a turbine generator.Such pressurized SOFC/turbine hybrid power generation systems providehigh electrical efficiency combined with low emissions. The turbine,which can be a micro-turbine (MT) or gas turbine (GT), serves both topressurize and provide air to the SOFC generator while the SOFCgenerator provides heat energy to operate the MT/GT generator. The poweroutput of the plant comprising the combination of the SOFC generator andthe MT/GT generator is the combined sum of the SOFC and MT/GT generatorelectrical outputs.

The SOFC and the MT/GT generators are highly coupled thermally andoperationally. Although several strategies for controlling the heatbalance of fuel cells and fuel cell generators are known, this couplinghas made it difficult to apply these strategies in SOFC/turbine hybridpower generation systems. Further complicating the control of thesepower generation systems is the fact that the outputs of the generatorsmust also be shared in a way that allows the following of a demandedload. These control requirements must be met at all times and mustrespond to outside disturbances and local transients.

BRIEF DESCRIPTION OF THE INVENTION

There is therefore provided, in some configurations of the presentinvention, a method for operating a pressurized solid oxide fuelcell/turbine (SOFC/turbine) hybrid power generation system in which thehybrid power generation system includes an SOFC generator and a turbinegenerator. The method includes controlling airflow to the SOFC/turbinehybrid power generation system in accordance with power demand andutilizing electrical current drawn from the SOFC generator to regulateSOFC generator temperature.

Some configurations of the present invention provide a power generationsystem that includes an SOFC hybrid plant having a pressurized SOFCgenerator, an unreacted fuel combustor, and a turbine generator. TheSOFC and turbine generator are configured to provide a summed poweroutput of the SOFT hybrid plant. Also provided is a system controllerconfigured to meet load requirements of the SOFC hybrid plant whilemaintaining temperature constraints on the SOFC generator and theunreacted fuel combustor.

Also, some configurations of the present invention provide a systemcontroller for a hybrid power plant that has an SOFC generator, aturbine generator, and an unreacted fuel combustor. The systemcontroller is configured to cool the SOFC generator to maintain apredetermined SOFC temperature setpoint and to maintain a surge marginduring airflow changes of the turbine generator.

It will be appreciated that various configurations of the presentinvention provide controlled, coordinated operation of a SOFC and aturbine generator to follow a demanded load while also providingsatisfactory thermal management.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram representative of a configuration of a powergeneration system of the present invention.

FIG. 2 is a graph showing the relationship of current density vs. powerdensity in one power supply configuration of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In some configurations of the present invention and referring to FIG. 1,a solid oxide fuel cell (SOFC)/turbine hybrid power generation system 10is provided in which control of both solid oxide fuel cell (SOFC)generator 14 temperature and total power system electrical power areprovided simultaneously. In these configurations, hybrid powergeneration system 10 is able to follow a demanded electrical poweroutput 28 while providing control of system thermal management andefficiency.

In some configurations of the present invention, a pressurizedSOFC/turbine power generation system 10 integrates a solid oxide fuelcell generator 14 with a turbine generator that includes a turbinegenerator 16 such as a micro-turbine (MT) or gas turbine (GT) in ahybridized power plant 12 that achieves high electrical efficiencycombined with low emissions. Turbine 16 serves both to pressurize andprovide air to SOFC generator 14 while the latter provides heat energyto operate turbine generator 16. SOFC generator 14 and turbine generator16 are highly coupled thermally and operationally. Thus, a systemcontroller 20 that implements a control strategy is provided in someconfigurations of the present invention to coordinate their operation.Plant power output 18 is the combined sum of SOFC generator 14 andturbine generator 16 electrical outputs. Outputs of SOFC generator 14and turbine generator 16 are shared in a way that allows following of ademanded load and thermal management of the hybrid power generationplant 12, which is part of hybrid power generation system 10. Thecontrol requirements are met at all times and respond to outsidedisturbances and load transients.

More particularly, configurations of the present invention allowpressurized SOFC/turbine hybrid power generations systems to stablyfollow a demanded system power setpoint 28 while maintaining keyparameters within specified limits. SOFC generator 14 operates within anarrow temperature window for both reliability and efficiency. A systemcontroller 20 is provided to balance the needs of SOFC generator 14 andthe power output demanded by a customer or customers. For example,reactants, temperatures, pressure, and/or component electrical outputare regulated. In some configurations, fuel flow, airflow, SOFCgenerator fuel utilization, MT/GT seed, MT/GT power, and/or SOFCgenerator current are controlled. Airflow and MT/GT generator poweroutput are related, so that, in some configurations, the power splitbetween SOFC generator 14 and turbine generator 16 are constrained bythermal management of SOFC generator 14. SOFC generator 14 power is astrong function of the electrical current demanded as well as fuelutilization, temperature and pressure.

System controller 20 is provided in some configurations of the presentinvention to successfully meet load requirements while maintainingtemperature constraints on the combustion of unutilized SOFT generator14 fuel in a combustor, such as unreacted fuel combustor (UFC) 22. Insome configurations of the present invention, lower level control loops38 are provided and are implemented by PI controllers or other suitablefeedback control structures.

In some configurations and referring again to FIG. 1, an airflowcontroller 24 limits the response time of the system to thereby maintainsufficient MT/GT compressor surge margin during airflow changes. Airflowcontroller 24 is provided because of the surge margin needed for ahybrid plant 12 having the addition of large volumes for SOFC generator14, manifolding, and other thermal management components (not shown inthe Figures) placed between an MT/GT compressor discharge and anexpander inlet (also not shown in the Figures). System controller 20includes, in some configurations of the present invention:

(1) an airflow setpoint controller 26 configured to determine an airflowsetpoint as a function of system power demand 28;

(2) an airflow controller 24 configured to determine (i.e., control) aturbine rotational speed dependent upon the airflow setpoint;

(3) a turbine speed controller 30 configured to determine and issue anMT/GT generator power command to thereby satisfy the required rotationalspeed;

(4) an SOFC generator current controller 32 configured to determine andissue a current command for SOFC generator 14, dependent upon theairflow, to thereby cool SOFC generator 14 to maintain a predeterminedSOFC temperature setpoint 34;

(5) a fuel utilization controller 36 configured to determine a fuelflow, dependent upon the current demand, to meet a desired fuel cellpower, while maintaining temperature constraints on the combustion ofunutilized SOFC generator fuel in combustor 22; and

(6) lower level control loops 38 configured to regulate lower levelcontrol functions such as the control of temperature bypass valves, fuelcontrol valves, power electronics, etc.

Airflow to SOFC hybrid power generation plant 12 in some configurationsis controlled using electrical current drawn from SOFC generator 14 toregulate the temperature of SOFC generator 14. The airflow is deliveredaccording to the power demand and is mapped as a function of operatingtemperature.

The power output of SOFC generator 14 is the product of the electricalcurrent drawn and SOFC generator 14 operating voltage. Thus, the voltagein some configurations of the present invention is trimmed so that thatthe total plant power output 18 matches the demanded power output. Fuelutilization controller 36 is used as a trim to vary the SOFC fuel flowto output the desired system power. SOFC generator 14 power trim isprovided in some configurations of the present invention by varying thefuel utilization about a nominal value to either increase or decreasethe operating voltage of SOFC generator 14.

System level optimization is performed in some configurations of thepreset invention. FIG. 2 is a graph of power density vs. currentdensity, showing operating lines 100, 102, and 104 drawn for SOFCgenerator 14 for various fuel utilizations in one configuration of thepresent invention. An optimization is performed in some configurationsthat balances current drawn with fuel utilization to maximize, or atleast increase, fuel efficiency and control temperature concurrentlywith the delivery of a constant power output. For example, if therepresented configuration is operating at the current density and fuelutilization represented by statepoint 106, the temperature of SOFCgenerator 14 is increased (because heat generated by SOFC generator 14is a strong function of current density) by moving the current densityand fuel utilization setpoints to those represented by statepoint 108.The system user will continue to receive the same amount of power as isdemanded, but SOFC generator 14 will move about its performanceoptimization surface. If optimization requires a still higher SOFCgenerator 14 temperature, the current density and fuel utilizationsetpoints are increased to those represented by setpoint 110. Byoperating in this manner and continuing to move about the constant SOFCgenerator load demand line, the SOFC/turbine hybrid power generationsystem configuration represented by FIG. 1 optimizes its performance.

It will thus be appreciated that configurations of the present inventionare able to follow a demanded electrical power output while providingcontrol of system thermal management and efficiency.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method for operating a pressurized solid oxide fuel cell/turbine(SOFC/turbine) hybrid power generation system comprising a solid oxidefuel cell (SOFC) generator and a turbine generator, said methodcomprising: controlling airflow to the SOFC/turbine hybrid powergeneration system in accordance with power demand; and utilizingelectrical current drawn from the SOFC generator to regulate SOFCgenerator temperature.
 2. A method in accordance with claim 1 whereinsaid controlling airflow to the SOFC/turbine hybrid generation powersystem further comprises controlling an airflow setpoint as a functionof power demand.
 3. A method in accordance with claim 2 wherein saidcontrolling airflow to the SOFC/turbine hybrid power generation systemfurther comprises controlling a turbine rotational speed dependent uponthe airflow setpoint.
 4. A method in accordance with claim 1 wherein theSOFC/turbine hybrid power generation system further comprises usingelectrical current drawn from the SOFC generator to control cooling ofthe SOFC generator to thereby maintain a predetermined SOFC temperaturesetpoint.
 5. A method in accordance with claim 1 further comprisingcontrolling the airflow in accordance with the power demand.
 6. A methodin accordance with claim 1 further comprising controlling a fuel flow,dependent upon a current demand, to meet a desired SOFC power.
 7. Amethod in accordance with claim 1 further comprising varying SOFC fuelflow to trim output voltage so that a total plant power output matches ademanded power output.
 8. A method in accordance with claim 1 whereinsaid controlling airflow to the SOFC/turbine hybrid power generationsystem in accordance with power demand further comprises controlling anairflow setpoint as a function of power demand and controlling a turbinerotational speed dependent upon the airflow setpoint and said methodfurther comprises cooling the SOFC generator to maintain a predeterminedSOFC temperature setpoint.
 9. A power generation system comprising: asolid oxide fuel cell (SOFC) hybrid plant comprising a pressurized solidoxide fuel cell (SOFC) generator, an unreacted fuel combustor, and aturbine generator, said SOFC generator and turbine generator, whereinsaid SOFC generator and said turbine generator are configured to providea summed power output of said SOFC hybrid plant; and a system controllerconfigured to meet load requirements of said SOFC hybrid plant whilemaintaining temperature constraints on said SOFC generator and saidunreacted fuel combustor.
 10. A power generation system in accordancewith claim 9 wherein said system controller further comprises an airflowcontroller configured to limit response time of said power generationsystem to thereby maintain a surge margin during airflow changes of saidturbine generator.
 11. A power generation system in accordance withclaim 10 wherein said controller further comprises an airflow setpointcontroller configured to control an airflow setpoint as a function ofpower demand.
 12. A power generation system in accordance with claim 11wherein said system controller further comprises a turbine speedcontroller configured to control said turbine generator to satisfy arotational speed requirement.
 13. A power generation system inaccordance with claim 9 wherein said system controller further comprisesan SOFC current controller configured to control cooling of said SOFCgenerator to maintain a predetermined SOFC temperature setpoint.
 14. Apower generation system in accordance with claim 13 wherein said systemcontroller further comprises a fuel utilization controller configured tocontrol a fuel flow, dependent upon current demand, to meet a desiredSOFC generator power.
 15. A power generation system in accordance withclaim 9 wherein said system controller comprises: an airflow controllerconfigured to limit response time of said power generation system tothereby maintain a surge margin during airflow changes of said turbinegenerator; an airflow setpoint controller configured to control anairflow setpoint as a function of power demand; a turbine speedcontroller configured to control said turbine generator to satisfy arotational speed requirement; an SOFC current controller configured tocool said SOFC generator to maintain a predetermined SOFC temperaturesetpoint; and a fuel utilization controller configured to control a fuelflow, dependent upon current demand, to meet a desired SOFC generatorpower.
 16. A system controller for a hybrid power generation planthaving a solid oxide fuel cell (SOFC) generator, a turbine generator,and an unreacted fuel combustor, said system controller configured tocool the SOFC generator to maintain a predetermined SOFC temperaturesetpoint and to maintain a surge margin during airflow changes of theturbine generator.
 17. A system controller in accordance with claim 16wherein said system controller comprises an airflow controllerconfigured to limit a response time of the hybrid power generationplant.
 18. A system controller in accordance with claim 17 wherein saidsystem controller further comprises an airflow setpoint controllerconfigured to control an airflow setpoint as a function of power demand.19. A system controller in accordance with claim 18 wherein said systemcontroller further comprises a turbine speed controller configured tocontrol the turbine generator to satisfy a rotational speed requirement.20. A system controller in accordance with claim 19 wherein said systemcontroller further comprises a fuel utilization controller configured tocontrol a fuel flow, dependent upon current demand, to meet a desiredfuel cell power.